1. Field of the Invention
This invention relates an improved system and method for reproducing an image, and more particularly to a system and method of reproducing gray levels in an electrostatic printing apparatus.
2. Description of Related Art
A typical reproduction machine scans and partitions the image to be copied into mutually exclusive areas called "pixels," each having a size corresponding to the spatial sampling resolution of the input section of the reproduction machine. The typical reproduction machine is capable of sensing a range of intensity values for each pixel, 0-16 gray levels for example. The typical reproduction machine output section, however, is capable of printing only a limited number of levels, black and white for example.
If the viewer of a reproduction were presented with only two levels, although the reproduction of text and many line drawings might be adequate, the reproduction of photographs or other images containing gray levels would not be adequate.
FIG. 1 shows a method of presenting gray levels to the viewer in a typical reproduction system. The image to be reproduced is partitioned into multiple mutually exclusive areas called "halftone cells," each containing a number of pixels. The size of the sample area of the input section of the reproduction system corresponds to a halftone cell, while the size of the printing area in the output section of the reproduction system corresponds to a pixel. Gray levels are simulated by turning on a subset of the total number of pixels in a halftone cell. The pixels are too small for the viewer to perceive the printing of individual pixels at a normal viewing distance, and the viewer instead perceives a gray level corresponding to the percentage of pixels that are turned on.
FIG. 1 shows seventeen different halftone cells corresponding to the range of gray levels between black (0) and white (16). To print black, all pixels in the halftone cell are turned on as shown at 105 in FIG. 1. To simulate an intermediate level of gray for the printing of a certain cell, half the pixels might be turned on as shown at level 8 110 in FIG. 1.
A fundamental drawback of the halftone cell technique illustrated in FIG. 1 is a trade-off that exists between number of gray levels and resolution. Since the area of the input sample is not the area of printing resolution, a pixel, but is instead the larger halftone cell area, resolution is reduced. Reducing the size of the halftone cell to include fewer pixels will increase resolution while reducing the number of gray levels.
Compounding the problem of loss of resolution is the nonlinear response of the human eye, which typically requires a compensating bias in the reproduction process. The human eye does not respond linearly to light intensity but instead responds approximately logarithmically. Thus, some of the gray levels implemented in FIG. 1 would not in fact be useful, because the eye would perceive little difference between those levels and the adjacent levels.